Modify properties in the .blend file (command line)
The following command line will do the job (-P will run the given python script before rendering):

blender -b OG1.blend  -P ChangeColor.py -o Render.png -f 0


ChangeColor.py:

import bpy

bpy.data.materials["NameOfMaterial"].diffuse_color = (0, 1.0, 0)


The value ranges for the color channels are [0 to 1.0].

Thank you for your help, poor! Blender ist just too awesome! &lt;3